1. Field of the Invention
The present invention relates to a method and apparatus for identifying the distribution of dielectric constants in an object employing microwave radiation and providing a visual presentation of the object.
2. Description of the Prior Art
A method for identifying the distribution of dielectric constants in an object is generally described in IEEE Transactions on Medical Imaging, Vol. MI-2, No. 4, December 1983, at pages 176-195. In this method, radiation is emitted by a microwave transmitter and irradiates an object to be examined. Radiation passing through and scattered by the object is measured in terms of amplitude and phase by a microwave detector. The distribution of the dielectric constants in the object is calculated from the output signal of the detector, and is forwarded to a means for visually displaying an image of the object.
Various efforts have been undertaken to provide a satisfactory method for identifying the distribution of dielectric constants .epsilon. in biological tissue, particularly in the human body. These efforts are based on the perception that the physical quantity .epsilon., critical for the interaction of electromagnetic radiation with biological tissue, can be used for imaging in medical diagnostics.
Early efforts in diffraction tomography have used either stationary linear antenna arrays, as described in "Limitations of Imaging With First-Order Diffraction Tomography," Slaney et al, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT 32, 1984, at pages 860-874 and in "Distortion In Diffraction Tomography Caused By Multiple Scattering," Azimi et al, IEEE Transactions on Medical Imaging, Vol. MI-2, No. 4, December 1983, at pages 176-195, or have used stationary planar arrays, as described in "A Microwave Diffraction Tomography System for Biomedical Applications," Peronnet et al, Proceedings of the 13th European Microwave Conference, September, 1983, pages 529-533, and in "Three-D Holographic Device Images RF Data Collected Once Over Entire Body," Pearce et al, RNM Images, April 1984, pages 32-33, 37 for acquiring the measured values from the transmitted radiation. In the systems described in the first three of the above articles, planar waves must be employed for irradiating the object. The images produced with these known systems are tomographs. In the system described in the last of the above articles, however, three-dimensional representations are attempted. In the systems in all of the articles, however, image reconstruction is undertaken with the assumption that the scattered field is generated by single scattering in the object being examined.
The four imaging systems described in the above-cited articles can therefore be employed only to a limited degree for medical diagnostic purposes, because those systems generate significantly falsified images of the .epsilon. distribution in biological tissue. One reason for this is that the assumption of single scattering (corresponding to the first Born approximation for the electromagnetic wave equation) is valid only for weakly scattering objects which are substantially homogeneous, and even under such conditions this assumption is only approximate. As is well known from the literature, both requirements (weak scattering and substantial homogeneity) are not characteristics of biological tissue.
The aforementioned articles in RNM Images discloses a measuring system wherein a plurality of individual detectors are arranged on a cylinder. These individual detectors are connected to a processing system for processing the measured data. Such a system requires a large number of individual detectors, and thus a corresponding number of processing channels, and is thus relatively complicated.
A holographic measuring system for radio-frequency is described in U.S. Pat. No. 3,887,923 wherein a plurality of antennas or individual detectors are also utilized. In this system, however, stationary antennas are again used. The system described therein is thus restricted to a linear array.
A method for recording acoustic, synthetic or microwave holograms is described in U.S. Pat. No. 3,818,427. In this method, a transmitter disposed in a container emits radiation in the direction of an object also in the container, and the resulting signal is sensed at the surface of liquid contained within the container. Sensing is undertaken by an individual (discrete) antenna. No three dimensional representation is possible with this method.